1. Field of the Invention
The present invention relates to electrolytic capacitors and more particularly to an electrolyte containing an indicator dye and a method for detection of electrolyte leakage and electrolyte pH utilizing the electrolyte.
2. Background Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, a cathode electrode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
The need for high voltage, high energy density capacitors is most pronounced when employed in implantable cardiac defibrillators (ICDs). In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode flat, stacked capacitor configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. In order to achieve higher energy densities, it has been necessary to stack three, four and five anodes per layer.
In an implantable cardioverter device, one concern is leakage of electrolyte from the electrolytic capacitors within the finished device. Electrolyte can leak through the weld zones between the housing and lid of the capacitor or through the feed through and up the anode wire. Leakage of the electrolyte negatively affects the performance of the capacitor resulting in a drop in the amount of energy delivered. Visual inspection is the current method for detection, however this process is difficult and time consuming because current electrolytes are transparent or a pale yellow in color, making it difficult to visually detect leaks. There remains a need in the art for an improved electrolyte leakage detection method that minimizes the difficulty of inspection and improves the speed and accuracy of detection.